[llvm] [INSTR COMBINE] implement combine select into cast (PR #114612)

[Stefan Gränitz via llvm-commits]

weliveindetail wrote:

> But even before that, wasn't the consensus from https://github.com/llvm/llvm-project/issues/107034 that this is better handled in the backend on the SDAG level, rather than in InstCombine?

That's right, they noted it in the ticket. Maybe the comments here can help finding a better place for it? https://github.com/llvm/llvm-project/pull/107732/files#r1749823010

FYI @Cr0a3 was asking for a good first issue and we spotted this one quickly. After all, it has that label. Not sure it's still the case if it must be implemented in ISel? Anyway, good first attempt :)

https://github.com/llvm/llvm-project/pull/114612